This invention relates to a method for preparing a purified dianhydride. More particularly, the method relates to preparing a purified oxydiphthalic anhydride.
Dianhydrides are often prepared from their chloro derivatives of corresponding anhydride by coupling two molecules of the chloro anhydride, generally through use of an additional coupling reagent. The crude product of such a coupling reaction often includes organic solvents, unreacted starting material(s) and catalysts, ionic substances, and various colored materials of unknown composition.
Various processes of the purification of phthalic anhydride and pyromellitic acid via activated carbon are disclosed in U.S. Pat. Nos. 1,301,388; 2,937,189; 2,985,665; 3,236,885; and 3,236,885. U.S. Pat. No. 2,786,805 teaches that phthalic anhydride can be purified by slurrying the material in water, heating the slurry to 375-400 F, removing the anhydride by passing steam into the mixture and condensing the purified phthalic anhydride vapors. While U.S. Pat. No. 3,338,923 discloses a method of purifying pyromellitic dianhydride by treatment with ketones. Furthermore, it discloses that the material can be purified by converting the dianhydride into the acid with water and recrystallizing the acid from water in the presence of activated carbon. In U.S. Pat. No. 4,870,194, oxydiphthalic anhydride can be purified by filtering or centrifuging a hot solution of the material in a high boiling solvent to remove impurities, followed by cooling the solution to precipitate the oxydiphthalic anhydride, which then can be isolated from the solution by filtration or centrifugation.
U.S. Pat. No. 4,906,760 discloses that metal ion impurities may be removed from aromatic anhydrides by refluxing the material in an aqueous solution, clarify the solution with activated carbon, filtering off the carbon, cooling and crystrallizing the purified aromatic acid. U.S. Pat. No. 4,906,760 discloses that metal ion impurities may be removed from aromatic anhydrides “by refluxing the anhydride in an aqueous solution, provide an activated adsorption agent such as activated carbon to clarify the solution, filtering off the absorption agent (and recovering the polyacid therefrom by washing the filter cake with warm water for return to the main solution), allowing the solution to stand and cool and precipitate the purified polyacid.
A British Patent, #823,507, discloses that tetrachlorophthalic acid can be purified by dissolving it in water containing 2-20% of a water miscible ether. The crude tetrachlorophthalic anhydride is dissolved in a mixture of water and the ether and then filtered hot.
U.S. Pat. No. 4,914,231 discloses a method for purifying diphenylsulfone tetracarboxylic acids by dissolution in a mixture of water and acetic acid to generate the crude tetracarboxylic acid and allowing the acid to crystallize. When the crude diphenylsulfone tetracarboxylic acid contains heavy metal ions, removal of the ions is more effective if the solution is treated with cation exchange resin, or with oxalic acid prior to crystallization. European Pat. No. 0421 046 A1 discloses a process for producing highly pure 3,3′,4,4′-biphenyltetracarboxylic acid or its respective anhydride. The anhydride is treated with hot water at a temperature of 95 to 105° C. The impurities dissolve in the water and the anhydride is converted to the acid.
U.S. Pat. No. 5,145,971 discloses a process for the preparation of purified oxydiphthalic acid from impure oxydiphthalic anhydride, by treating with a mixture of water and propionic acid or butyric acid to produce oxydiphthalic acid. The acid may be treated to reform oxydiphthalic anhydride. The method for generating oxydiphthalic anhydride from oxydiphthalic acid has been disclosed in U.S. Pat. No. 5,336,788 where the acid is mixed with an organic solvent, followed by addition of propionic acid and co-distillation of the acid with water, and heating the resulting slurry to the boiling point of the slurry to remove any water.
However there is a continuing need to develop an improved process to isolate and purify the dianhydrides so as to remove the impurities such as those salt-by product, residual catalyst and some residual reactants that tend to contaminate the dianhydride. It would be desirable therefore to provide such a method for preparing a purified dianhydride devoid of contaminations.